Isotopic and Trace Element Effects of Crustal Assimilation and Slab Contributions in Island Arc Magmas, Grenada, Lesser Antilles

M. F. Thirlwall Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK

A. M. Graham BP Exploration Operation Co. Ltd., Fairburn Industrial Estate, Dyce, Aberdeen AB2 0PB, UK

R. J. Arculus Department of Geology, Australian National University, ACT0200, Australia

New Pb, Sr, Nd and O isotope and chemical data are presented for the picrite to low-CaO andesite differentiation series (the M-series) of Grenada, Lesser Antilles island arc. Revised isotopic data for the low-Mg, high-Ca C-series basalts show tight correlations with MgO that strongly support published interpretations that the C-series evolved by crustal assimilation coupled with fractional crystallisation (AFC). The most magnesian C-series basalts can be generated by about 16% olivine fractionation from picrites that were very different from the observed M-series picrites, with greater large ion lithophile element (LILE) enrichment and more MORB-like isotopic ratios. The strong enrichment in LILE relative to rare earths, and in rare earths relative to Nb and Zr, implies fluid transport of these elements from subducted altered ocean crust. An origin in the subducted crust for the Sr enrichment is consistent with constant 87Sr/86Sr around 0.7045, although lower 143Nd/144Nd and higher Th/U than MORB requires a small additional subducted sediment contribution, also apparent in 207Pb/204Pb elevation above the Northern Hemisphere Reference Line. Some LILE (Pb, Rb) are not strongly enriched, and Ce/Pb ratios are remarkably close (20-45) to those of MORB (ca. 25). The fluid released from the slab, despite in part being derived from subducted sediment, must have MORB-like Ce/Pb. This may result from release of a substantial part of the slab Pb at shallower depth than the zone of magma generation. Grenada andesites (>58% SiO2) lie on extensions of C-series AFC correlations, suggesting that they were generated by continued amphibole-dominated fractionation from C-series parents. They never achieve strongly radiogenic Pb, most probably because Pb is incompatible, while Nd has high bulk distribution coefficients due to apatite and hornblende fractionation. AFC thus results in progressively slower 206Pb/204Pb increase as magmatic Pb contents increase.

Higher 206Pb/204Pb and Pb/Nd in M-series lavas are thought to be largely controlled by high-level crustal assimilation, mostly through AFC processes. Two sub-divisions of the M-series are recognised with lower and higher La/Y: 143Nd/144Nd is near-constant and higher (0.512874-0.512904) in the low-La/Y group, and lower and more varied in the high-La/Y group (0.51283-0.51256), but the two groups show similar ranges in 87Sr/86Sr and 206Pb/204Pb. The highest 206Pb/204Pb (ca. 19.7) and Pb/Nd is found in the three basic andesites of the low-La/Y group. These however have quite high 143Nd/144Nd (0.51281), and it is impossible to generate these compositions by mixing local seafloor sediments into depleted mantle. The occurrence of high-d18O (+10”) quartz xenocrysts in two of the three samples, coexisting with phenocryst olivine, requires that the high-206Pb/204Pb component is of shallow crustal origin, perhaps being carbonate-rich sediments. At constant 143Nd/144Nd, both high- and low-La/Y groups vector toward this crustal component, suggesting 2-6% bulk crustal assimilation can explain the Pb and Sr isotopic variability of the M-series, and probably also their lower Ce/Pb ratios. Conversely, the most LREE-enriched picrite has the lowest 206Pb/204Pb and 143Nd/144Nd, which is consistent with generation from a mantle source with a relatively high contribution (ca. 2%) from subducted local seafloor sediment (206Pb/204Pb ca. 19.2). Progressively more contribution from this sediment can successfully explain the Nd isotopic shift from the C-series through the low-La/Y M-series picrites to the high-La/Y M-series picrites. Nd isotopic compositions appear to be largely controlled by subducted sediment, while Pb compositions and Ce/Pb ratios are controlled by shallow crustal assimilation. The extent of fluid modification of the mantle source, as monitored by LILE enrichment relative to LREE, and LREE enrichment relative to HFSE, broadly decreases as the subducted sediment component increases.